The present invention relates generally to wireless communications and in particular to a technique for implementing private emergency call services.
Trends in wireless communication systems such as cellular mobile telephone systems, Personal Communication Services (PCS) systems, and the like, for the most part continue to suggest digital techniques are preferred for radio modulation. Although competing standards have been implemented in certain areas, the Global System for Mobile (GSM) communication is the most widely implemented wireless system in the world.
The architecture of a GSM system can be broadly divided into three subsystems, including a Mobile Station, a Base Station Subsystem, and a Network Sub-System. The mobile station (MS) is the portable subscriber or user equipment with which most people are familiar. The Base Station Subsystem (BSS) is typically considered to be composed of two parts, a Base Transceiver Station (BTS) and a Base Station Controller (BSC). The BTS typically houses radio transceivers that are used to communicate signals over the air within a particular cell and handles the radio interface protocols (referred to as the Um protocols in GSM terminology) that permit the mobile station to communicate with the BSS. The BSC manages the radio resources for one or more BTS's across an interface known as the Abis interface. This functionality is concerned with managing the available radio channels such as set up, tear down, frequency assignment, and the like, as well as the required handover processing needed to continue a call in progress as a mobile station moves from cell to cell.
The central component of a GSM system is the Mobile Switching Center (MSC). The MSC acts as a switching node of a Public Switched Telephone Network (PSTN), but also provides other services needed in a mobile telephone system such as subscriber registration, authentication, location update, inter-MSC handovers, and call routing to permit subscriber roaming. These services are provided in conjunction with four intelligent databases known as the Home Location Register (HLR), Visitor Location Register (VLR), Equipment Identity Register (EIR), and Authentication Center (AuC). These databases form the remainder of the so-called Network Sub-System (NSS) and perform administrative functions such as obtaining the location of subscribers within the network, roaming administration for mobile stations located in cells which are not part of their home area, equipment authentication and security, as well as user authentication.
Like other modern communication systems, a GSM network makes use of Open Systems Interconnect (OSI) type layered protocol to define messaging and various other functionalities. The Layer 1 (LI) or physical layer of a GSM system is generally considered to be the aforementioned Um interface defining specific radio signalling between the MS and BTS. Over the land network, such as between the BTS and BSC, L1 may be implemented with standard telephone cabling, optic fiber microwave transmission, or other physical media used for moving digital telephony signals.
Layer 2 (L2) of GSM is a data link layer responsible for the packaging of data to be transmitted. The most widespread method for such framing is a high level data link control (HDLC) protocol known as LAPD.
Of specific concern to the present invention is the Layer 3 (L3) or network layer of GSM. This layer prescribes the path a message needs to take and who the recipient of that message is, maintaining all information necessary to route a packet to its intended destination.
Within Layer 3, the GSM standard further defines functional layers which perform certain tasks known as Radio Resource management (RR), Mobility Management (MM), and Connection Management (CM). The RR functional layer is responsible for providing a reliable radio link between the MS and the network infrastructure. This includes the establishment and allocation of radio channels, as well as performing handover processing, an essential element of cellular systems. Handover processing involves coordination of the mobile station, the BSS, and even the MSC to some degree.
The MM functional layer of Layer 3 within GSM assumes a reliable RR connection. The MM functional layer is responsible for location management and security. Location management involves procedures for updating the position of a mobile station so that the mobile station's current location is always properly stored at the HLR. This permits incoming calls to be properly routed. The MM layer also involves functions required for authentication of the mobile to prevent unauthorized access to the network, using the AuC database.
The CM functional layer is divided into Call Control (CC), Supplementary Services (SS), and Short Message Service (SMS) sublayers. Basically, the first two sublayers are responsible for routing, establishment, maintenance and release of calls. The third sublayer also provides certain supplementary services, such as a short message service that delivers data messages.